Tuning a Nitro Engine with a 3 needle Carburetor, by a Factory YS test pilot

As Bob mentioned this was a spur the moment type deal. Now that I've had some time to gather my thoughts, I'd like to clarify a couple points I was trying to get across in the video series....This is long overdue.

There are many different methods for tuning a Nitro Engine. Understand that certain flying styles demand "Tweaks" here and there. But the basic method of tuning is the same, for all engines with a 3 needle Carburetor. Most engine manufacturers have recommended factory settings to get the engine to run. Use these settings to start. But understand this is a broad recommendation. Where you fly, the fuel you run, the glow plug you use, headspeed, gear ratio, type of governor, pitch range, and pilot all effect the engines tune.

The method I've developed over the past 4 years of flying Nitro powered 3d-helicopters, is based on real life experience, and the practical application of the knowledge I gained along the way. This is just what works for me, and gets me consistent results. There are many view points, methods, and opinions on the subject. Smarter people than I could talk circles around me on the Theory of engine tuning. Theory is useful to grasp the concept of what we are trying to do, but it ultimatly comes down to the real world application of flying the helicopter. To me, above all, the end result is what counts, and what I strive for.

Quick note: It is absolutely vital to make certain both the Fuel system, and exhaust system are “tight” and leak free. We are attempting to Very very finely adjust an air/fuel mixture with a carburetor here. Any “un-metered” air being introduced to the engine outside of the carburetor, we can not control, and is going to cause erratic behavior. All of the tuning efforts we make will be nearly useless, if we have ANY leaks! The carburetor can only do its job if it has 100% control of the air/fuel mixture! I can not emphasize enough how important this is!

With that said, and before we get to turning any needles, We first need to understand the basic relationship of the 3 needle valves and what areas of "Throttle range" they effect:

The High end needle valve:
This controls the amount of fuel allowed at Wide open throttle.

The mid-range needle valve:
This controls the "Ramp" between the idle circuit and full throttle. It is adding fuel across this entire Range. As you progressively open the throttle barrel, this needle is adding fuel as needed. When the barrel reaches the full open position, the high end needle takes over completely. For these reasons the mid-range needle is often referred to as the “transition” needle. In order to get consistent power, these two needles need to “mesh” with one another. With the ultimate goal of giving the engine fuel, only when and where it needs it.

The Idle circuit controls just that, the mixture at idle.

Before we get to the practical application to the above theory, lets take some time to understand what we are looking for, when we say “Rich” or “Lean”

This is difficult to put into words so bare with me...This is much easier to demonstrate than explain through type:

“Rich”
Lots of thick/heavy smoke.
“Blubbering/unclean sound”
Raw Liguid fuel “spitting” out the exhaust pipe
The motor never seems to “get on the Pipe” or “clean up” and get out of the way of itself...
It struggles to RPM.

“Lean”
Inconsistent Thin smoke
Raspy, ping, ping, noise when under load
The engine will feel like its making great power, and then fall off and start to “sag” or “bog” once you put a consistent load on it.
The engine sounds sick/laboring really hard
Can produce over-reving high pitch type sound

Keeping that in mind. Here is my personal method, that I use on a day to day basis, in this specific order:

Remember to always tune Rich to Lean, not the other way around.

Tune the High end needle first:
Do something like a Full pitch Climb out/Big hurricanes/Full throttle loops/Full pitch FFF/ Anything that's full wide open throttle. To accurately test this, we need to “clean out” any residual fuel in the engine from an excessively Rich, or out of tune mid-range....this was illustrated in one of the videos. To do so, get the engine at Wide open throttle and keep it there for 10 seconds or so.

Start/Find Rich, Lean 2-3 clicks at a time until you find lean, Then Richen it back up 3 clicks. Don't be afraid to “find” lean, this will not harm the engine what so ever, just don't keep it there! Find it, then back off. But we have to find it in order to know where we are.

Once we've done the above the High end needle should be close.

Now Tune the mid-range needle:
Do something continuous, I prefer Elevator Tic tocs/Full cyclic flips.....Try and load it up and see how the engine responds. How it responds is going to determine what we do with the mid-range needle.

If the engine sounds like its "blubbering" and just doesn't sound clean and "on the pipe" its too Rich. If it goes into the maneuver kickin ass and then starts to fall off/lose power its too lean.

Again tune Rich to lean here. Find Rich, tune to find lean, then back off 2-3 clicks.

Once that's done, re-visit the high end adjustment. It should still be very close, minor 1-2 click adjustment to get it just right. Once satisfied with high end performance once more, the Engine should have a “baseline” tune. And the two main needle valves should be close to “synched”

Now we get to tune the idle mixture. The key here is to make sure the throttle barrel Is only open far enough to be on the “idle circuit” too far open and we are getting into the mid-range needle. And your hopes to get a consistent idle go out the window. Tune the idle just like you would the others. If the engine blubbers, and generally struggles to idle its too rich. If the idle surges up/down, is inconsistent and makes a sound like....bing..ba..bing...bing its too lean. Leave this needle at factory settings (generally 1 turn) until the rest of the engines range is tuned.

Here is where we can stop, and call it “good enough” This will get you a pretty happy engine that will satisfy most.

However, If your looking to get the very last edge of Power, one has to combine the above methods. Do continuous tic tocs, then immediately and without hesitation, go into wide open throttle type flight. If you “breathe” the engine between the transition from tic tocs to wide open throttle, the test becomes no good. This is pretty difficult to do! What your listening for here is how the engine responds through the transition into wide open throttle, if it stutters, then quickly picks up, the relationship between the two needles is too lean. If it blubbers, its too rich. You're also looking for a consistent, smooth smoke trail exiting the exhaust pipe. Along with “feeling” if the power curve is linear.

And remember on all the above, the “mid-range” needle is whats controlling this transition. The “high end” is simply controlling wide open throttle. Blend the “mid-range” transition into the “high end” mixture, not the other way around!

Its important to take this step by step and get the two needles “synched” If the two needles are not in line with one another, there is very little hope the engine will be happy throughout its range, and it will have “lean spots” across its power curve.

Tuning is an art. It can be frustrating to learn, and its unforgiving. But its quite satisfying when you finally get it right. I don't believe it can be taught. Rather learned through raw self experience. This “guide” should help get ya going the right direction. But ultimately its trial and error in order to learn it, and ultimately get it right. You have to tune for your abilities and flying style. My tune is not equal to your tune, or anyone elses. Each pilot needs to tune to their own specific setup and flying ability. They may not all be the same, but they can all be tuned well with the above method.

Thank you for this great series. For nitro newbs this is truly priceless info. I had always herd that if you run your engine lean even for a few sec. Thats it, and you have started some process of premature engine failure?

Tim I was wondering if i could get your opinion on the method I was taught to tune the YS idle needle.

Find the needle setting were the engine RPM peaks. So i VERY SLOWLY lean the idle screw, the RPM will continue to increase. I make sure to do this very slowly, sometimes I will have to adjust my throttle trim (a sign that it was too high to begin with). Eventually I will find a point that if I continue to lean the idle needle the RPM will actually drop. The needle setting where it peaked is my peak RPM point. I then add 1/8th turn to richen up the idle needle.

Firstly Bob - that was a nice set of vids, and is a well overdue addition to all your other brill 101's. I started out in this hobby back in early 2008 and without your 101's Im sure that I would still be scratching my head.

On the whole I personally dont say to much here on Helifreak - to be honest I dont ever feel that Im experienced enough to comment to much? I know thats stupid - but thats the way I am. However on this occassion - I feel that I have to submit my appreciation at the amazing flying that Tim demonstrated (specially the 1st Vid in the Fog) - just totally amazing. I love that "FAST - BIG AIR" style of flying. How can flying get any better than that? JUST SUPERB and it looked as if you werent even trying!!?

Tim - I hate to think what you're like if your ever angry!!!

Just one question that Ive been meaning to ask for ages - do you think there is likely to be any future (radical) development with Nitro powered flight to really keep pace with the - power/performance of electric?? It seems to me that electric is quietly advancing - but Nitro just seems to sit there doing the same old thing - but is that view point wrong??